Move pixel config getter to private in GrBackendSurface

We eventually want to remove pixel config from GrBackendSurface so this
helps to insure that clients don't rely on it.

Bug: skia:
Change-Id: I6b8435d12347fab62c0f9032addea1211aa703ca
Reviewed-on: https://skia-review.googlesource.com/70642
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
2 files changed